This invention relates generally to scroll type compressors and more specifically to a scroll type compressor having provision for the injection of liquid refrigerant at an intermediate stage of the compression cycle to thereby reduce overheating.
Scroll compressors are known to be extremely efficient, reliable and quiet in applications for the compression of refrigerant. However, like all compressors, they are subject to overheating during certain high load situations.
In the normal refrigeration cycle, vapor is drawn into a compressor where it is compressed to a higher pressure. The compressed vapor is cooled and condensed in a 15 condenser into a high pressure liquid which is then expanded, typically through an expansion valve, to a lower pressure and caused to evaporate in an evaporator to thereby draw in heat and thus provide the desired cooling effect. The expanded, relatively low pressure vapor exiting the evaporator is once again drawn into the compressor and the cycle starts anew. The action of compressing the vapor imparts work onto the vapor and results in a significant increase in the vapor temperature. While a substantial portion of this heat is subsequently rejected to the atmosphere during the condensation process, a portion of the heat is transferred to the compressor components. Depending upon the specific refrigerant vapor compressed and on the pressure conditions of operation, this heat transfer can cause the temperature of the compressor components to rise to levels which may cause the compressor to overheat, resulting in degradation of the compressor performance and lubrication and possible damage to the compressor.
In order to overcome overheating problems, various methods have been developed for injecting gaseous or liquid refrigerant under pressure into the suction inlet of a compressor where it expands and cools the inlet vapor and the compression chamber. Two such system are disclosed and described in detail in commonly assigned U.S. Pat. Nos. 5,076,067 and 4,974,427 the disclosures of which are hereby expressly incorporated herein by reference. However, injecting refrigerant into the inlet port of the compressor suffers the disadvantage that it reduces the compressor efficiency by reducing the net amount of refrigerant drawn into the compressor on the suction side of the refrigeration circuit. In order to minimize this reduction in efficiency, systems have been developed using thermostats or other thermal transducer circuits incorporating valve means to limit the injection of refrigerant to only those times when the compressor temperature rises to a certain preset temperature, such as occurring under abnormally high load situations. Other methods of controlling the amount of liquid injection include providing capillary tubes or thermal expansion valves. While these devices are simple and relatively low cost, they are known to leak excess refrigerant from the high pressure discharge side into the relatively low pressure suction side of the compressor, thus potentially increasing flooding problems. Additionally, when the compressor is deactivated, high pressure refrigerant can further migrate through these devices to the normally low pressure inlet of the compressor, thus increasing the chance of starting problems.
Another known system reduces discharge temperature by injecting liquid refrigerant directly into the pumping chamber at an intermediate pressure point therein. The disadvantage of such a system is that it requires very accurate, repeatable and long life thermostatic devices, as well as reliable, long life control valves. Substantial extra machining is also required.
The present invention overcomes the aforesaid disadvantages of prior liquid injection systems by providing a system which is self-regulating and therefore eliminates the complexity introduced by thermostat control systems and which provide for the injection of liquid refrigerant into an existing chamber in many scroll machines which is always adjacent to and in fluid communication with an intermediate stage of the compressor; i.e. the intermediate axial biasing chamber for enhancing scroll tip sealing. In addition, a restriction is provided to reduce the pressure of the injected liquid to approximately that of the intermediate stage of the compressor. In this arrangement, the increase or decrease in pressure at the intermediate stage of the compressor in response to increase or decrease of suction pressure, and hence the pressure differential across the compressor, acts to automatically regulate the amount of liquid refrigerant injected, thus providing enough liquid to cool the compressor without causing flooding. Further, the present invention provides for an optional simple valve actuated in response to operation of the compressor to prevent migration of fluid into the compressor when it is not operating. The present invention also contemplates the use of bleed hole pairs (symmetrical or preferably non-symmetrical) for the injection of liquid refrigerant, without any type of intermediate pressure axial biasing. The term "liquid injection" is used herein to denote that it is liquid refrigerant which is taken from downstream of the condenser, but in reality a small portion of this liquid is vaporized as it flows to and into the compressor so that it is a two phase (liquid and vapor) fluid which is actually injected into the compressor. This is to be distinguished from vapor injection systems where pure vapor is taken from a heat exchanger or subcooler and is introduced into the compressor at an intermediate pressure.
Theoretically, there is no thermodynamic advantage (or penalty) to be derived from the use of liquid injection into an intermediate pressure-compression chamber for the purpose of discharge gas cooling. On the other hand, because a real system is not perfect in the theoretical sense, it has been observed that some heat transfer inefficiencies are in fact reduced in the compressor super heat process by the injection of liquid refrigerant, and as a consequence efficiency increases of 2 to 4 percent can be realized.
The present invention is uniquely adaptable to provide cooling by injecting liquid refrigerant into intermediate axial pressure biasing chambers on either the non-orbiting scroll side or the orbiting scroll side of the compressor, and/or through unequally located bleed holes.